A User Equipment (UE), also known as a mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication network, sometimes also referred to as a cellular radio system. The communication may be made, e.g., between UEs, between a UE and a wire connected telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The wireless communication may comprise various communication services such as voice, messaging, packet data, video, broadcast, etc.
The UE may further be referred to as mobile telephone, cellular telephone, computer tablet or laptop with wireless capability, etc. The UE in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another UE or a server.
The wireless communication network covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station, e.g., a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used.
Sometimes, the expression “cell” may be used for denoting the radio network node itself. However, the cell may also in normal terminology be used for the geographical area where radio coverage is provided by the radio network node at a base station site. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes may communicate over the air interface operating on radio frequencies with any UE within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the UE. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the UE to the radio network node.
Beyond 3G mobile communication systems, such as e.g., 3GPP LTE, offer high data rate in the downlink by employing Multiple-Input and Multiple-Output (MIMO) with Orthogonal Frequency Division Multiplexing (OFDM) access scheme at the UE.
A UE, before being able to receive downlink data from a serving radio network node, has to make a channel estimation. The channel estimation is based on a reference signal emitted by the radio network node. A number of reference signals have been defined in the LTE downlink, e.g., Cell-specific Reference Signal (CRS). CRS is transmitted in all subframes and in all resource blocks of the carrier.
In an effort to increase the flexibility of the radio network node's spectrum occupancy, various new waveforms have been proposed in the literature recently such as e.g. Filter Bank Multi-Carrier (FBMC) modulation, and/or precoded OFDM as notable examples. Typically, radio network nodes employing these waveforms distinguish themselves over today's established classical OFDM transmitters in that they move the functionality of prior art classical, low-pass spectrum-shaping transmitter-filters into the transmitter baseband modulator itself. By doing so, these radio network nodes combine extremely low out-of-band emission with a flexibility to rapidly change spectrum occupancy. By these virtues, they are considered very suitable for future radio systems that build on emerging concepts as dynamic, flexible spectrum access and spectrum sharing.
One such approach is based on precoding the data symbols prior to the OFDM modulation step typically in a K-point Discrete Fourier Transform (DFT).
FIG. 1A is illustrating an example of spectral precoding concept in a transmitter, which transmitter may be comprised in a radio network node for downlink transmission, or in a UE for uplink transmission. In each of the versions of this precoding approach as known in the prior art, data symbols are slightly corrupted for the sake of low out-of-band emissions. The symbols that modulate the subcarriers φk(t) of an OFDM symbol are collected in a vector d=[d0 d1 d2 . . . dK-1]T and have the general form:d=Gd  [Equation 1]
where d=[d0 d1 d2 . . . dD-1]T is the vector of D≦K information symbols and G is a K×D matrix denoting the spectral precoder. Further, the transmitted signal, generated by the K-point transmitter DFT is:S(t)=Σkdkφk(t)  [Equation2]
where φk(t) are the K subcarriers used in the DFT modulator and φk(t)=ej2πti(k)/T. Here, the index function i(k) maps each index in {0, 1, . . . , K−1} onto an arbitrary subcarrier index set {k0, k1, . . . , kK-1}. The index function i(k) may allow for an arbitrary order of the elements in the symbol vector d.
A receiver may then detect the information symbols d either by using knowledge of the actual precoding operation at the transmitter, knowledge the receiver has through a standard specification or through a signalling channel; or by simply ignoring the transmitter precoding operation and rely on the fact that the distortion incurred at the transmitter is so small that the receiver-performance is not significantly deteriorated.
Essentially, the known precoders are determined by forcing the precoded symbols d to satisfy a set of M linear constraints collected in one matrix equation:Ad=0  [Equation3]
where A is the size M×K constraint matrix, embodying the M linear constraints. The matrix A is chosen in such a way that the out-of-band emission of the signal s(t) is small.
Two kinds of the constraint matrices A are known, one based on a time-domain continuity argument, i.e. N-continuous multicarrier. The other one is based on a spectrum notching argument. For the problem addressed here, the actual choice of particular prior-art choice of constraint matrix is irrelevant.
Further, the literature explicitly describes two kinds of precoding structures, one where D=K (resulting in a class of projection precoders), and one where D<K, resulting in a class of orthogonal precoders. In the former, the price for reduced out-of-band emission is paid in terms of a reduced error-rate performance at the receiver. In the latter the price is paid in the form of slightly reduced data-rate and a higher implementation complexity at both transmitter and receiver. For the problem addressed here, the actual choice of particular precoder structure is transparent.
In classical OFDM typically certain subcarriers are modulated by pre-defined pilot symbols known at the receiver. FIG. 1B illustrates how for instance pilot symbols are scattered among the information-carrying data symbols in the time-frequency grid. Depending on the operation mode, i.e. the number of deployed antennas by the transmitter, the signal for each transmit antenna carries pilots, also referred to as reference symbols, located on some standard-defined subcarriers that depend on the antenna-index (labelling the transmit-antennas) and also on a time index (labelling the 14 OFDM symbols in a timeslot-pair).
A receiver typically uses its knowledge of pilot symbols (besides the locations, the actual symbol-values are standard-defined and hence known at the receiver) to estimate how the radio channel has changed/affected the transmitted signal's phase and amplitude at the respective pilot positions in the time-frequency grid. In a second step, the receiver then typically interpolates these channel effects in order to estimate how the radio channel affects the transmitted signal at positions in the time-frequency-grid where data symbols are transmitted. This information is crucial for the reliable detection of the data symbols.
One problem with the precoding structure illustrated in FIG. 1A is that the precoder corrupts any OFDM pilot symbols, different from one symbol to another, and dependent on the information symbols transmitted on information-carrying subcarriers. This distortion appears in a way that is unpredictable by the receiver. Hence, a receiver channel estimation procedure would exhibit deteriorated performance, which, in turn, would lead to a degradation of the error-rate/throughput performance of the communication link.
Further, certain subcarriers may be subject to a backwards compatibility requirement in the sense that traditional OFDM receivers may perceive these subcarriers as if no precoding were applied in the transmitter. In a system with prior-art spectral precoding such backwards-compatible receivers would exhibit reduced performance because they operate as if no transmit-precoding is performed, and the actual transmit precoding appears as an additional signal distortion. For instance, in legacy systems, certain subcarriers may contain control signals, synchronization signals, broadcast channels, and/or basically any other reference signal in a system such as e.g. CSI-RS, DM-RS, PRS in LTE-terminology.
In addition, certain subcarriers may be used by the transmitter to transmit “side information” about the actual precoder used. If some knowledge about the transmit-precoder operation is not available to the receiver by other means e.g. through a standard-specification, or through a separate, other, communication channel, the multicarrier transmitter may convey this information on some subcarriers of the precoded signal itself. However, as also the subcarriers transmitting the side information concerning the used precoder are precoded, it is not possible for the receiver to decode the side information reliably.
In short, pre-coding of pilots or reference symbols render distortion of these signals which makes channel estimation at the receiver side difficult or even impossible.
Further, prior art precoding have problems with backward compatibility, as existing standards may not be capable of dealing with information symbols comprised in precoded signals.
In addition, information symbols carrying essential side information about the precoder cannot be detected reliably by the receiver, without the knowledge of the essential side information.
In a multi-antenna system this problem occurs on each antenna.